The present invention relates to a liquid crystal display apparatus using a liquid crystal modulation element such as a liquid crystal projector and performing liquid crystal drive control such as overdrive for improving moving image display performance.
Some of the liquid crystal modulation elements (also called as liquid crystal display elements) are realized by sealing nematic liquid crystal having positive dielectric anisotropy between a first transparent substrate having a transparent electrode (common electrode) formed thereon and a second transparent substrate having a transparent electrode (pixel electrode) forming pixels, wiring, switching elements and the like formed thereon.
The liquid crystal modulation element is referred to as a Twisted Nematic (TN) liquid crystal modulation element in which the major axes of liquid crystal molecules are twisted by 90 degrees continuously between the two glass substrates. This liquid crystal modulation element is used as a transmissive liquid crystal modulation element. Some of the liquid crystal modulation elements utilize a circuit substrate having reflecting mirrors, wiring, switching elements and the like formed thereon instead of the abovementioned second transparent substrate. This is called a Vertical Aligned Nematic (VAN) liquid crystal modulation element in which the major axes of liquid crystal molecules are alignment in homeotropic alignment substantially perpendicularly to two substrates. The liquid crystal modulation element is used as a reflective liquid crystal modulation element.
In these liquid crystal modulation elements, typically, Electrically Controlled Birefringence (ECB) effect is used to provide retardation for a light wave passing through a liquid crystal layer to control the change of polarization of the light wave, thereby forming an image from the light.
In the liquid crystal modulation element, which utilizes the ECB effect to modulate the light intensity, application of an electric field to the liquid crystal layer moves ionic materials present in the liquid crystal layer. When a DC electric field is continuously applied to the liquid crystal layer, the ionic materials are pulled toward one of two opposite electrodes. Even when a constant voltage is applied to the electrodes, the electric field applied to the liquid crystal layer is cancelled out by the charged ions to substantially attenuate the electric field applied to the liquid crystal layer.
To avoid such a phenomenon, a line inversion drive method is typically employed in which the polarity of an applied electric field is reversed between positive and negative for each line of arranged pixels and is changed in a predetermined cycle such as 60 Hz or the like. In addition, a field inversion drive method is used in which the polarity of an applied electric field to all of arranged pixels is reversed between positive and negative in a predetermined cycle. Those drive methods can avoid the application of the electric field of only one polarity to the liquid crystal layer to prevent the unbalanced ions. This corresponds to controlling the effective electric field to be applied to the liquid crystal layer such that it always has the same value as the voltage to be applied to the electrodes.
So-called overdrive has been known as a drive method for the purpose of improving the display quality of the liquid crystal modulation element. In the overdrive, when the liquid crystal modulation element is driven so as to display a moving image whose tone (or tone value) changes with time, the tone values of two field images that are temporally continuous are compared. When the tone value increases, the liquid crystal modulation element is driven with an increased tone value that is higher than an original display tone value. When the tone value decreases on the other hand, the liquid crystal modulation element is driven with a decreased tone value that is lower than the original display tone value. The use of such overdrive as described above improves the response speed of the liquid crystal in a halftone (middle tone) display state, and thereby blur of a displayed moving image is reduced.
The overdrive of the liquid crystal modulation element has been disclosed in, for example, Japanese Patent Laid-Open No. 2001-034238 (Japanese Patent No. 3407698).
However, the overdrive to display the moving image on the liquid crystal modulation element for a long time results in application of a DC voltage component to a liquid crystal layer thereof in average. This is because the absolute values of liquid crystal applied electric fields (hereinafter also simply referred to as voltages) corresponding to positive and negative overdrive amounts in a certain tone are unbalanced.
For example, a case is assumed where a black display state and a certain halftone display state are cyclically switched. In this case, the voltage corresponding to a certain overdrive amount is applied to the liquid crystal layer in the switching from the black display state in which no voltage is applied to the liquid crystal layer to the halftone display state. On the other hand, the voltage corresponding to the overdrive amount is zero in the switching from the halftone display state to the black display state. When such unbalanced voltages applied to the liquid crystal layer are frequently caused in, for example, moving image display performed by continuously scanning a stripe pattern image, and then the voltage component corresponding to the difference of the unbalanced voltages is accumulated, the DC voltage component is applied to the liquid crystal layer.
In a conventional direct-view-type liquid crystal panel, line inversion drive is employed in which voltages having opposite polarities to each other are applied to each of adjacent lines of display electrodes formed in the liquid crystal modulation element as a countermeasure against the application of the DC voltage component to the liquid crystal layer. Alternatively, dot inversion drive is also employed where voltages having opposite polarities to each other are applied to each of adjacent pixels.
These drive methods can balance out the DC voltage components in the adjacent lines or pixels.
In a liquid crystal display apparatus such as an image projection apparatus using a micro display, however, the line inversion drive and the dot inversion drive cause an abnormal alignment of the liquid crystal which provides an adverse influence on a displayed image. To prevent this, the field inversion drive is recently used in which one field is driven with a single polarity. However, the field inversion drive cannot suppress the application of the DC voltage component to the liquid crystal layer in the overdrive.
Japanese Patent No. 3407698 has disclosed a method that appropriate selection of the material of the electrodes can solve a problem in which a so-called “stain” caused due to the application of the DC voltage component to the liquid crystal layer in the overdrive.
However, the problem caused by the application of the DC voltage component to the liquid crystal layer is not limited to the “stain” described in Japanese Patent No. 3407698. Specifically, burn-in or flicker is also caused. Thus, the application of the DC voltage component to the liquid crystal layer must be prevented essentially.